Rotating shaft designs for bearing cartridges to be used in the assembly of motors, actuators and the like are highly desirable, because the shaft can frequently be made smaller, requiring less power to rotate the shaft. Such cartridges as previously developed typically have a spindle mounted by means of two ball bearing systems to a shaft disposed in the center of the hub. One of the bearings is typically located near the top of the spindle and the other near the bottom. These bearings allow for rotational movement between the shaft and the hub while maintaining accurate alignment of the hub to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional ball-bearing system described above is prone, however, to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings run under conditions that generally result in some measure of physical contact between raceways and balls, this in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally even and smooth, but microscopically uneven and rough raceways, transmit this surface structure as well as their imperfections in sphericity in the form of vibration to the shaft. This vibration limits the overall performance of the disc drive system.
Another problem is related to the damage due to shocks to the bearing cartridge in turn show up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material and leave permanent deformation and damage on raceways and balls.
Moreover, mechanical bearings are not always scalable to smaller dimensions, a problem in many rotating shaft mechanical systems which are being scaled down in size while operated at high speeds and greater rates of acceleration.
As an alternative to conventional ball bearing spindle systems, researchers are developing a hydrodynamic bearing. In these types of systems, lubricating fluid--either gas or liquid--functions as the actual bearing surface between a stationary base or housing and the rotating spindle or rotating hub and the stationary surrounding portion of the motor. For example, liquid lubricants comprising oil, more complex ferromagnetic fluids, or even air have been utilized for use in hydrodynamic bearing systems. The reason for the popularity of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the head disc housing. However, air does not provide the lubricating qualities of oil. Its low viscosity requires smaller bearing gaps and therefore higher tolerance standards to achieve similar dynamic performance.
Many prior art hydrodynamic bearing assemblies frequently require large or bulky structural elements for supporting the axial and radial loads, as such hydrodynamic bearings do not have the inherent stiffness which results from mechanical bearing assemblies. It is difficult to scale the structural support elements to fit within smaller disc drive dimensions currently in consumer demand, as well as other small scale mechanical systems. In other instances, hydrodynamic bearing assemblies suffer from the disadvantages of requiring extremely tight clearances and alignments; this burden makes it difficult to manufacture such assemblies since even a small deviation or aberration can lead to faulty bearings.
Most known hydrodynamic bearing designs are based on a fixed shaft and rotating surrounding sleeve. However, by switching to a rotating shaft, significant improvements in power consumption and vibration response could be achieved with no trade-offs in performance. The power consumption would be decreased by using a smaller diameter shaft which has a smaller oil shearing radius, thus requiring less torque to rotate. This smaller diameter shaft can be used because the stability of the design is less dependent on the shaft stiffness than when the shaft is stationary and cantilevered or supported from the base.
The vibration performance could also be improved significantly in a rotating shaft design when the angular stiffness of the base-shaft system increases when it is replaced by a base-sleeve system. A design utilizing a fixed sleeve supported from a base has a much higher angular stiffness than one using a shaft cantilevered from a base. Therefore, the development of an easily assembled rotating shaft cartridge incorporating a fluid dynamic bearing is highly desirable.